The carboxyhaemoglobin (COHb) levels of two groups of workers in the City of London were determined. The mean COHb in smokers (5-8%) was significantly greater than that of the non-smoking group (1-3%). In general, cigarette consumption could be directly related to the COHb level but there was considerable individual variation. The mean COHb level of manual workers was significantly lower than that of sedentary workers even though the former group tended to smoke more heavily.